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7c673cae FG |
1 | ================== |
2 | Placement Groups | |
3 | ================== | |
4 | ||
11fdf7f2 TL |
5 | .. _pg-autoscaler: |
6 | ||
7 | Autoscaling placement groups | |
8 | ============================ | |
9 | ||
10 | Placement groups (PGs) are an internal implementation detail of how | |
11 | Ceph distributes data. You can allow the cluster to either make | |
12 | recommendations or automatically tune PGs based on how the cluster is | |
13 | used by enabling *pg-autoscaling*. | |
14 | ||
15 | Each pool in the system has a ``pg_autoscale_mode`` property that can be set to ``off``, ``on``, or ``warn``. | |
16 | ||
17 | * ``off``: Disable autoscaling for this pool. It is up to the administrator to choose an appropriate PG number for each pool. Please refer to :ref:`choosing-number-of-placement-groups` for more information. | |
18 | * ``on``: Enable automated adjustments of the PG count for the given pool. | |
19 | * ``warn``: Raise health alerts when the PG count should be adjusted | |
20 | ||
21 | To set the autoscaling mode for existing pools,:: | |
22 | ||
23 | ceph osd pool set <pool-name> pg_autoscale_mode <mode> | |
24 | ||
25 | For example to enable autoscaling on pool ``foo``,:: | |
26 | ||
27 | ceph osd pool set foo pg_autoscale_mode on | |
28 | ||
29 | You can also configure the default ``pg_autoscale_mode`` that is | |
30 | applied to any pools that are created in the future with:: | |
31 | ||
9f95a23c | 32 | ceph config set global osd_pool_default_pg_autoscale_mode <mode> |
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33 | |
34 | Viewing PG scaling recommendations | |
35 | ---------------------------------- | |
36 | ||
37 | You can view each pool, its relative utilization, and any suggested changes to | |
38 | the PG count with this command:: | |
39 | ||
40 | ceph osd pool autoscale-status | |
41 | ||
42 | Output will be something like:: | |
43 | ||
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44 | POOL SIZE TARGET SIZE RATE RAW CAPACITY RATIO TARGET RATIO EFFECTIVE RATIO PG_NUM NEW PG_NUM AUTOSCALE |
45 | a 12900M 3.0 82431M 0.4695 8 128 warn | |
46 | c 0 3.0 82431M 0.0000 0.2000 0.9884 1 64 warn | |
47 | b 0 953.6M 3.0 82431M 0.0347 8 warn | |
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48 | |
49 | **SIZE** is the amount of data stored in the pool. **TARGET SIZE**, if | |
50 | present, is the amount of data the administrator has specified that | |
51 | they expect to eventually be stored in this pool. The system uses | |
52 | the larger of the two values for its calculation. | |
53 | ||
54 | **RATE** is the multiplier for the pool that determines how much raw | |
55 | storage capacity is consumed. For example, a 3 replica pool will | |
56 | have a ratio of 3.0, while a k=4,m=2 erasure coded pool will have a | |
57 | ratio of 1.5. | |
58 | ||
59 | **RAW CAPACITY** is the total amount of raw storage capacity on the | |
60 | OSDs that are responsible for storing this pool's (and perhaps other | |
61 | pools') data. **RATIO** is the ratio of that total capacity that | |
62 | this pool is consuming (i.e., ratio = size * rate / raw capacity). | |
63 | ||
64 | **TARGET RATIO**, if present, is the ratio of storage that the | |
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65 | administrator has specified that they expect this pool to consume |
66 | relative to other pools with target ratios set. | |
67 | If both target size bytes and ratio are specified, the | |
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68 | ratio takes precedence. |
69 | ||
9f95a23c TL |
70 | **EFFECTIVE RATIO** is the target ratio after adjusting in two ways: |
71 | ||
72 | 1. subtracting any capacity expected to be used by pools with target size set | |
73 | 2. normalizing the target ratios among pools with target ratio set so | |
74 | they collectively target the rest of the space. For example, 4 | |
75 | pools with target_ratio 1.0 would have an effective ratio of 0.25. | |
76 | ||
77 | The system uses the larger of the actual ratio and the effective ratio | |
78 | for its calculation. | |
79 | ||
11fdf7f2 TL |
80 | **PG_NUM** is the current number of PGs for the pool (or the current |
81 | number of PGs that the pool is working towards, if a ``pg_num`` | |
82 | change is in progress). **NEW PG_NUM**, if present, is what the | |
83 | system believes the pool's ``pg_num`` should be changed to. It is | |
84 | always a power of 2, and will only be present if the "ideal" value | |
85 | varies from the current value by more than a factor of 3. | |
86 | ||
87 | The final column, **AUTOSCALE**, is the pool ``pg_autoscale_mode``, | |
88 | and will be either ``on``, ``off``, or ``warn``. | |
89 | ||
90 | ||
91 | Automated scaling | |
92 | ----------------- | |
93 | ||
94 | Allowing the cluster to automatically scale PGs based on usage is the | |
95 | simplest approach. Ceph will look at the total available storage and | |
96 | target number of PGs for the whole system, look at how much data is | |
97 | stored in each pool, and try to apportion the PGs accordingly. The | |
98 | system is relatively conservative with its approach, only making | |
99 | changes to a pool when the current number of PGs (``pg_num``) is more | |
100 | than 3 times off from what it thinks it should be. | |
101 | ||
102 | The target number of PGs per OSD is based on the | |
103 | ``mon_target_pg_per_osd`` configurable (default: 100), which can be | |
104 | adjusted with:: | |
105 | ||
106 | ceph config set global mon_target_pg_per_osd 100 | |
107 | ||
108 | The autoscaler analyzes pools and adjusts on a per-subtree basis. | |
109 | Because each pool may map to a different CRUSH rule, and each rule may | |
110 | distribute data across different devices, Ceph will consider | |
111 | utilization of each subtree of the hierarchy independently. For | |
112 | example, a pool that maps to OSDs of class `ssd` and a pool that maps | |
113 | to OSDs of class `hdd` will each have optimal PG counts that depend on | |
114 | the number of those respective device types. | |
115 | ||
116 | ||
117 | .. _specifying_pool_target_size: | |
118 | ||
119 | Specifying expected pool size | |
120 | ----------------------------- | |
121 | ||
122 | When a cluster or pool is first created, it will consume a small | |
123 | fraction of the total cluster capacity and will appear to the system | |
124 | as if it should only need a small number of placement groups. | |
125 | However, in most cases cluster administrators have a good idea which | |
126 | pools are expected to consume most of the system capacity over time. | |
127 | By providing this information to Ceph, a more appropriate number of | |
128 | PGs can be used from the beginning, preventing subsequent changes in | |
129 | ``pg_num`` and the overhead associated with moving data around when | |
130 | those adjustments are made. | |
131 | ||
9f95a23c TL |
132 | The *target size* of a pool can be specified in two ways: either in |
133 | terms of the absolute size of the pool (i.e., bytes), or as a weight | |
134 | relative to other pools with a ``target_size_ratio`` set. | |
11fdf7f2 TL |
135 | |
136 | For example,:: | |
137 | ||
138 | ceph osd pool set mypool target_size_bytes 100T | |
139 | ||
140 | will tell the system that `mypool` is expected to consume 100 TiB of | |
141 | space. Alternatively,:: | |
142 | ||
9f95a23c | 143 | ceph osd pool set mypool target_size_ratio 1.0 |
11fdf7f2 | 144 | |
9f95a23c TL |
145 | will tell the system that `mypool` is expected to consume 1.0 relative |
146 | to the other pools with ``target_size_ratio`` set. If `mypool` is the | |
147 | only pool in the cluster, this means an expected use of 100% of the | |
148 | total capacity. If there is a second pool with ``target_size_ratio`` | |
149 | 1.0, both pools would expect to use 50% of the cluster capacity. | |
11fdf7f2 TL |
150 | |
151 | You can also set the target size of a pool at creation time with the optional ``--target-size-bytes <bytes>`` or ``--target-size-ratio <ratio>`` arguments to the ``ceph osd pool create`` command. | |
152 | ||
153 | Note that if impossible target size values are specified (for example, | |
9f95a23c TL |
154 | a capacity larger than the total cluster) then a health warning |
155 | (``POOL_TARGET_SIZE_BYTES_OVERCOMMITTED``) will be raised. | |
156 | ||
157 | If both ``target_size_ratio`` and ``target_size_bytes`` are specified | |
158 | for a pool, only the ratio will be considered, and a health warning | |
159 | (``POOL_HAS_TARGET_SIZE_BYTES_AND_RATIO``) will be issued. | |
11fdf7f2 TL |
160 | |
161 | Specifying bounds on a pool's PGs | |
162 | --------------------------------- | |
163 | ||
164 | It is also possible to specify a minimum number of PGs for a pool. | |
165 | This is useful for establishing a lower bound on the amount of | |
166 | parallelism client will see when doing IO, even when a pool is mostly | |
167 | empty. Setting the lower bound prevents Ceph from reducing (or | |
168 | recommending you reduce) the PG number below the configured number. | |
169 | ||
170 | You can set the minimum number of PGs for a pool with:: | |
171 | ||
172 | ceph osd pool set <pool-name> pg_num_min <num> | |
173 | ||
174 | You can also specify the minimum PG count at pool creation time with | |
175 | the optional ``--pg-num-min <num>`` argument to the ``ceph osd pool | |
176 | create`` command. | |
177 | ||
7c673cae FG |
178 | .. _preselection: |
179 | ||
180 | A preselection of pg_num | |
181 | ======================== | |
182 | ||
183 | When creating a new pool with:: | |
184 | ||
9f95a23c | 185 | ceph osd pool create {pool-name} [pg_num] |
7c673cae | 186 | |
9f95a23c TL |
187 | it is optional to choose the value of ``pg_num``. If you do not |
188 | specify ``pg_num``, the cluster can (by default) automatically tune it | |
189 | for you based on how much data is stored in the pool (see above, :ref:`pg-autoscaler`). | |
7c673cae | 190 | |
9f95a23c TL |
191 | Alternatively, ``pg_num`` can be explicitly provided. However, |
192 | whether you specify a ``pg_num`` value or not does not affect whether | |
193 | the value is automatically tuned by the cluster after the fact. To | |
194 | enable or disable auto-tuning,:: | |
7c673cae | 195 | |
9f95a23c | 196 | ceph osd pool set {pool-name} pg_autoscale_mode (on|off|warn) |
7c673cae | 197 | |
9f95a23c TL |
198 | The "rule of thumb" for PGs per OSD has traditionally be 100. With |
199 | the additional of the balancer (which is also enabled by default), a | |
200 | value of more like 50 PGs per OSD is probably reasonable. The | |
201 | challenge (which the autoscaler normally does for you), is to: | |
7c673cae | 202 | |
9f95a23c TL |
203 | - have the PGs per pool proportional to the data in the pool, and |
204 | - end up with 50-100 PGs per OSDs, after the replication or | |
205 | erasuring-coding fan-out of each PG across OSDs is taken into | |
206 | consideration | |
7c673cae FG |
207 | |
208 | How are Placement Groups used ? | |
209 | =============================== | |
210 | ||
211 | A placement group (PG) aggregates objects within a pool because | |
212 | tracking object placement and object metadata on a per-object basis is | |
213 | computationally expensive--i.e., a system with millions of objects | |
214 | cannot realistically track placement on a per-object basis. | |
215 | ||
216 | .. ditaa:: | |
217 | /-----\ /-----\ /-----\ /-----\ /-----\ | |
218 | | obj | | obj | | obj | | obj | | obj | | |
219 | \-----/ \-----/ \-----/ \-----/ \-----/ | |
220 | | | | | | | |
221 | +--------+--------+ +---+----+ | |
222 | | | | |
223 | v v | |
224 | +-----------------------+ +-----------------------+ | |
225 | | Placement Group #1 | | Placement Group #2 | | |
226 | | | | | | |
227 | +-----------------------+ +-----------------------+ | |
228 | | | | |
229 | +------------------------------+ | |
230 | | | |
231 | v | |
232 | +-----------------------+ | |
233 | | Pool | | |
234 | | | | |
235 | +-----------------------+ | |
236 | ||
237 | The Ceph client will calculate which placement group an object should | |
238 | be in. It does this by hashing the object ID and applying an operation | |
239 | based on the number of PGs in the defined pool and the ID of the pool. | |
240 | See `Mapping PGs to OSDs`_ for details. | |
241 | ||
242 | The object's contents within a placement group are stored in a set of | |
243 | OSDs. For instance, in a replicated pool of size two, each placement | |
244 | group will store objects on two OSDs, as shown below. | |
245 | ||
246 | .. ditaa:: | |
7c673cae FG |
247 | +-----------------------+ +-----------------------+ |
248 | | Placement Group #1 | | Placement Group #2 | | |
249 | | | | | | |
250 | +-----------------------+ +-----------------------+ | |
251 | | | | | | |
252 | v v v v | |
253 | /----------\ /----------\ /----------\ /----------\ | |
254 | | | | | | | | | | |
255 | | OSD #1 | | OSD #2 | | OSD #2 | | OSD #3 | | |
256 | | | | | | | | | | |
257 | \----------/ \----------/ \----------/ \----------/ | |
258 | ||
259 | ||
260 | Should OSD #2 fail, another will be assigned to Placement Group #1 and | |
261 | will be filled with copies of all objects in OSD #1. If the pool size | |
262 | is changed from two to three, an additional OSD will be assigned to | |
263 | the placement group and will receive copies of all objects in the | |
264 | placement group. | |
265 | ||
11fdf7f2 | 266 | Placement groups do not own the OSD; they share it with other |
7c673cae FG |
267 | placement groups from the same pool or even other pools. If OSD #2 |
268 | fails, the Placement Group #2 will also have to restore copies of | |
269 | objects, using OSD #3. | |
270 | ||
271 | When the number of placement groups increases, the new placement | |
272 | groups will be assigned OSDs. The result of the CRUSH function will | |
273 | also change and some objects from the former placement groups will be | |
274 | copied over to the new Placement Groups and removed from the old ones. | |
275 | ||
276 | Placement Groups Tradeoffs | |
277 | ========================== | |
278 | ||
279 | Data durability and even distribution among all OSDs call for more | |
280 | placement groups but their number should be reduced to the minimum to | |
281 | save CPU and memory. | |
282 | ||
283 | .. _data durability: | |
284 | ||
285 | Data durability | |
286 | --------------- | |
287 | ||
288 | After an OSD fails, the risk of data loss increases until the data it | |
289 | contained is fully recovered. Let's imagine a scenario that causes | |
290 | permanent data loss in a single placement group: | |
291 | ||
292 | - The OSD fails and all copies of the object it contains are lost. | |
293 | For all objects within the placement group the number of replica | |
11fdf7f2 | 294 | suddenly drops from three to two. |
7c673cae | 295 | |
11fdf7f2 | 296 | - Ceph starts recovery for this placement group by choosing a new OSD |
7c673cae FG |
297 | to re-create the third copy of all objects. |
298 | ||
299 | - Another OSD, within the same placement group, fails before the new | |
300 | OSD is fully populated with the third copy. Some objects will then | |
301 | only have one surviving copies. | |
302 | ||
303 | - Ceph picks yet another OSD and keeps copying objects to restore the | |
304 | desired number of copies. | |
305 | ||
306 | - A third OSD, within the same placement group, fails before recovery | |
307 | is complete. If this OSD contained the only remaining copy of an | |
308 | object, it is permanently lost. | |
309 | ||
310 | In a cluster containing 10 OSDs with 512 placement groups in a three | |
311 | replica pool, CRUSH will give each placement groups three OSDs. In the | |
312 | end, each OSDs will end up hosting (512 * 3) / 10 = ~150 Placement | |
313 | Groups. When the first OSD fails, the above scenario will therefore | |
314 | start recovery for all 150 placement groups at the same time. | |
315 | ||
316 | The 150 placement groups being recovered are likely to be | |
317 | homogeneously spread over the 9 remaining OSDs. Each remaining OSD is | |
318 | therefore likely to send copies of objects to all others and also | |
319 | receive some new objects to be stored because they became part of a | |
320 | new placement group. | |
321 | ||
322 | The amount of time it takes for this recovery to complete entirely | |
323 | depends on the architecture of the Ceph cluster. Let say each OSD is | |
324 | hosted by a 1TB SSD on a single machine and all of them are connected | |
325 | to a 10Gb/s switch and the recovery for a single OSD completes within | |
326 | M minutes. If there are two OSDs per machine using spinners with no | |
327 | SSD journal and a 1Gb/s switch, it will at least be an order of | |
328 | magnitude slower. | |
329 | ||
330 | In a cluster of this size, the number of placement groups has almost | |
331 | no influence on data durability. It could be 128 or 8192 and the | |
332 | recovery would not be slower or faster. | |
333 | ||
334 | However, growing the same Ceph cluster to 20 OSDs instead of 10 OSDs | |
335 | is likely to speed up recovery and therefore improve data durability | |
336 | significantly. Each OSD now participates in only ~75 placement groups | |
337 | instead of ~150 when there were only 10 OSDs and it will still require | |
338 | all 19 remaining OSDs to perform the same amount of object copies in | |
339 | order to recover. But where 10 OSDs had to copy approximately 100GB | |
340 | each, they now have to copy 50GB each instead. If the network was the | |
341 | bottleneck, recovery will happen twice as fast. In other words, | |
342 | recovery goes faster when the number of OSDs increases. | |
343 | ||
344 | If this cluster grows to 40 OSDs, each of them will only host ~35 | |
345 | placement groups. If an OSD dies, recovery will keep going faster | |
346 | unless it is blocked by another bottleneck. However, if this cluster | |
347 | grows to 200 OSDs, each of them will only host ~7 placement groups. If | |
348 | an OSD dies, recovery will happen between at most of ~21 (7 * 3) OSDs | |
349 | in these placement groups: recovery will take longer than when there | |
350 | were 40 OSDs, meaning the number of placement groups should be | |
351 | increased. | |
352 | ||
353 | No matter how short the recovery time is, there is a chance for a | |
354 | second OSD to fail while it is in progress. In the 10 OSDs cluster | |
355 | described above, if any of them fail, then ~17 placement groups | |
356 | (i.e. ~150 / 9 placement groups being recovered) will only have one | |
357 | surviving copy. And if any of the 8 remaining OSD fail, the last | |
358 | objects of two placement groups are likely to be lost (i.e. ~17 / 8 | |
359 | placement groups with only one remaining copy being recovered). | |
360 | ||
361 | When the size of the cluster grows to 20 OSDs, the number of Placement | |
362 | Groups damaged by the loss of three OSDs drops. The second OSD lost | |
363 | will degrade ~4 (i.e. ~75 / 19 placement groups being recovered) | |
364 | instead of ~17 and the third OSD lost will only lose data if it is one | |
365 | of the four OSDs containing the surviving copy. In other words, if the | |
366 | probability of losing one OSD is 0.0001% during the recovery time | |
11fdf7f2 | 367 | frame, it goes from 17 * 10 * 0.0001% in the cluster with 10 OSDs to 4 * 20 * |
7c673cae FG |
368 | 0.0001% in the cluster with 20 OSDs. |
369 | ||
370 | In a nutshell, more OSDs mean faster recovery and a lower risk of | |
371 | cascading failures leading to the permanent loss of a Placement | |
372 | Group. Having 512 or 4096 Placement Groups is roughly equivalent in a | |
373 | cluster with less than 50 OSDs as far as data durability is concerned. | |
374 | ||
375 | Note: It may take a long time for a new OSD added to the cluster to be | |
376 | populated with placement groups that were assigned to it. However | |
377 | there is no degradation of any object and it has no impact on the | |
378 | durability of the data contained in the Cluster. | |
379 | ||
380 | .. _object distribution: | |
381 | ||
382 | Object distribution within a pool | |
383 | --------------------------------- | |
384 | ||
385 | Ideally objects are evenly distributed in each placement group. Since | |
386 | CRUSH computes the placement group for each object, but does not | |
387 | actually know how much data is stored in each OSD within this | |
388 | placement group, the ratio between the number of placement groups and | |
389 | the number of OSDs may influence the distribution of the data | |
390 | significantly. | |
391 | ||
11fdf7f2 | 392 | For instance, if there was a single placement group for ten OSDs in a |
7c673cae FG |
393 | three replica pool, only three OSD would be used because CRUSH would |
394 | have no other choice. When more placement groups are available, | |
395 | objects are more likely to be evenly spread among them. CRUSH also | |
396 | makes every effort to evenly spread OSDs among all existing Placement | |
397 | Groups. | |
398 | ||
399 | As long as there are one or two orders of magnitude more Placement | |
eafe8130 TL |
400 | Groups than OSDs, the distribution should be even. For instance, 256 |
401 | placement groups for 3 OSDs, 512 or 1024 placement groups for 10 OSDs | |
402 | etc. | |
7c673cae FG |
403 | |
404 | Uneven data distribution can be caused by factors other than the ratio | |
405 | between OSDs and placement groups. Since CRUSH does not take into | |
406 | account the size of the objects, a few very large objects may create | |
407 | an imbalance. Let say one million 4K objects totaling 4GB are evenly | |
eafe8130 | 408 | spread among 1024 placement groups on 10 OSDs. They will use 4GB / 10 |
7c673cae FG |
409 | = 400MB on each OSD. If one 400MB object is added to the pool, the |
410 | three OSDs supporting the placement group in which the object has been | |
411 | placed will be filled with 400MB + 400MB = 800MB while the seven | |
412 | others will remain occupied with only 400MB. | |
413 | ||
414 | .. _resource usage: | |
415 | ||
416 | Memory, CPU and network usage | |
417 | ----------------------------- | |
418 | ||
419 | For each placement group, OSDs and MONs need memory, network and CPU | |
420 | at all times and even more during recovery. Sharing this overhead by | |
421 | clustering objects within a placement group is one of the main reasons | |
422 | they exist. | |
423 | ||
424 | Minimizing the number of placement groups saves significant amounts of | |
425 | resources. | |
426 | ||
11fdf7f2 TL |
427 | .. _choosing-number-of-placement-groups: |
428 | ||
7c673cae FG |
429 | Choosing the number of Placement Groups |
430 | ======================================= | |
431 | ||
11fdf7f2 TL |
432 | .. note: It is rarely necessary to do this math by hand. Instead, use the ``ceph osd pool autoscale-status`` command in combination with the ``target_size_bytes`` or ``target_size_ratio`` pool properties. See :ref:`pg-autoscaler` for more information. |
433 | ||
7c673cae FG |
434 | If you have more than 50 OSDs, we recommend approximately 50-100 |
435 | placement groups per OSD to balance out resource usage, data | |
11fdf7f2 | 436 | durability and distribution. If you have less than 50 OSDs, choosing |
7c673cae | 437 | among the `preselection`_ above is best. For a single pool of objects, |
f67539c2 | 438 | you can use the following formula to get a baseline |
7c673cae | 439 | |
f67539c2 | 440 | Total PGs = :math:`\frac{OSDs \times 100}{pool \: size}` |
7c673cae FG |
441 | |
442 | Where **pool size** is either the number of replicas for replicated | |
443 | pools or the K+M sum for erasure coded pools (as returned by **ceph | |
444 | osd erasure-code-profile get**). | |
445 | ||
446 | You should then check if the result makes sense with the way you | |
447 | designed your Ceph cluster to maximize `data durability`_, | |
448 | `object distribution`_ and minimize `resource usage`_. | |
449 | ||
eafe8130 TL |
450 | The result should always be **rounded up to the nearest power of two**. |
451 | ||
452 | Only a power of two will evenly balance the number of objects among | |
453 | placement groups. Other values will result in an uneven distribution of | |
454 | data across your OSDs. Their use should be limited to incrementally | |
455 | stepping from one power of two to another. | |
7c673cae FG |
456 | |
457 | As an example, for a cluster with 200 OSDs and a pool size of 3 | |
f67539c2 | 458 | replicas, you would estimate your number of PGs as follows |
7c673cae | 459 | |
f67539c2 | 460 | :math:`\frac{200 \times 100}{3} = 6667`. Nearest power of 2: 8192 |
7c673cae FG |
461 | |
462 | When using multiple data pools for storing objects, you need to ensure | |
463 | that you balance the number of placement groups per pool with the | |
464 | number of placement groups per OSD so that you arrive at a reasonable | |
465 | total number of placement groups that provides reasonably low variance | |
466 | per OSD without taxing system resources or making the peering process | |
467 | too slow. | |
468 | ||
469 | For instance a cluster of 10 pools each with 512 placement groups on | |
470 | ten OSDs is a total of 5,120 placement groups spread over ten OSDs, | |
471 | that is 512 placement groups per OSD. That does not use too many | |
472 | resources. However, if 1,000 pools were created with 512 placement | |
473 | groups each, the OSDs will handle ~50,000 placement groups each and it | |
474 | would require significantly more resources and time for peering. | |
475 | ||
224ce89b WB |
476 | You may find the `PGCalc`_ tool helpful. |
477 | ||
478 | ||
7c673cae FG |
479 | .. _setting the number of placement groups: |
480 | ||
481 | Set the Number of Placement Groups | |
482 | ================================== | |
483 | ||
484 | To set the number of placement groups in a pool, you must specify the | |
485 | number of placement groups at the time you create the pool. | |
11fdf7f2 | 486 | See `Create a Pool`_ for details. Even after a pool is created you can also change the number of placement groups with:: |
7c673cae FG |
487 | |
488 | ceph osd pool set {pool-name} pg_num {pg_num} | |
489 | ||
11fdf7f2 | 490 | After you increase the number of placement groups, you must also |
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491 | increase the number of placement groups for placement (``pgp_num``) |
492 | before your cluster will rebalance. The ``pgp_num`` will be the number of | |
493 | placement groups that will be considered for placement by the CRUSH | |
494 | algorithm. Increasing ``pg_num`` splits the placement groups but data | |
495 | will not be migrated to the newer placement groups until placement | |
496 | groups for placement, ie. ``pgp_num`` is increased. The ``pgp_num`` | |
497 | should be equal to the ``pg_num``. To increase the number of | |
498 | placement groups for placement, execute the following:: | |
499 | ||
500 | ceph osd pool set {pool-name} pgp_num {pgp_num} | |
501 | ||
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502 | When decreasing the number of PGs, ``pgp_num`` is adjusted |
503 | automatically for you. | |
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504 | |
505 | Get the Number of Placement Groups | |
506 | ================================== | |
507 | ||
508 | To get the number of placement groups in a pool, execute the following:: | |
509 | ||
510 | ceph osd pool get {pool-name} pg_num | |
511 | ||
512 | ||
513 | Get a Cluster's PG Statistics | |
514 | ============================= | |
515 | ||
516 | To get the statistics for the placement groups in your cluster, execute the following:: | |
517 | ||
518 | ceph pg dump [--format {format}] | |
519 | ||
520 | Valid formats are ``plain`` (default) and ``json``. | |
521 | ||
522 | ||
523 | Get Statistics for Stuck PGs | |
524 | ============================ | |
525 | ||
526 | To get the statistics for all placement groups stuck in a specified state, | |
527 | execute the following:: | |
528 | ||
529 | ceph pg dump_stuck inactive|unclean|stale|undersized|degraded [--format <format>] [-t|--threshold <seconds>] | |
530 | ||
531 | **Inactive** Placement groups cannot process reads or writes because they are waiting for an OSD | |
532 | with the most up-to-date data to come up and in. | |
533 | ||
534 | **Unclean** Placement groups contain objects that are not replicated the desired number | |
535 | of times. They should be recovering. | |
536 | ||
537 | **Stale** Placement groups are in an unknown state - the OSDs that host them have not | |
538 | reported to the monitor cluster in a while (configured by ``mon_osd_report_timeout``). | |
539 | ||
540 | Valid formats are ``plain`` (default) and ``json``. The threshold defines the minimum number | |
541 | of seconds the placement group is stuck before including it in the returned statistics | |
542 | (default 300 seconds). | |
543 | ||
544 | ||
545 | Get a PG Map | |
546 | ============ | |
547 | ||
548 | To get the placement group map for a particular placement group, execute the following:: | |
549 | ||
550 | ceph pg map {pg-id} | |
551 | ||
552 | For example:: | |
553 | ||
554 | ceph pg map 1.6c | |
555 | ||
556 | Ceph will return the placement group map, the placement group, and the OSD status:: | |
557 | ||
558 | osdmap e13 pg 1.6c (1.6c) -> up [1,0] acting [1,0] | |
559 | ||
560 | ||
561 | Get a PGs Statistics | |
562 | ==================== | |
563 | ||
564 | To retrieve statistics for a particular placement group, execute the following:: | |
565 | ||
566 | ceph pg {pg-id} query | |
567 | ||
568 | ||
569 | Scrub a Placement Group | |
570 | ======================= | |
571 | ||
572 | To scrub a placement group, execute the following:: | |
573 | ||
574 | ceph pg scrub {pg-id} | |
575 | ||
576 | Ceph checks the primary and any replica nodes, generates a catalog of all objects | |
577 | in the placement group and compares them to ensure that no objects are missing | |
578 | or mismatched, and their contents are consistent. Assuming the replicas all | |
579 | match, a final semantic sweep ensures that all of the snapshot-related object | |
580 | metadata is consistent. Errors are reported via logs. | |
581 | ||
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582 | To scrub all placement groups from a specific pool, execute the following:: |
583 | ||
584 | ceph osd pool scrub {pool-name} | |
585 | ||
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586 | Prioritize backfill/recovery of a Placement Group(s) |
587 | ==================================================== | |
588 | ||
589 | You may run into a situation where a bunch of placement groups will require | |
590 | recovery and/or backfill, and some particular groups hold data more important | |
591 | than others (for example, those PGs may hold data for images used by running | |
592 | machines and other PGs may be used by inactive machines/less relevant data). | |
593 | In that case, you may want to prioritize recovery of those groups so | |
594 | performance and/or availability of data stored on those groups is restored | |
11fdf7f2 | 595 | earlier. To do this (mark particular placement group(s) as prioritized during |
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596 | backfill or recovery), execute the following:: |
597 | ||
598 | ceph pg force-recovery {pg-id} [{pg-id #2}] [{pg-id #3} ...] | |
599 | ceph pg force-backfill {pg-id} [{pg-id #2}] [{pg-id #3} ...] | |
600 | ||
601 | This will cause Ceph to perform recovery or backfill on specified placement | |
602 | groups first, before other placement groups. This does not interrupt currently | |
603 | ongoing backfills or recovery, but causes specified PGs to be processed | |
604 | as soon as possible. If you change your mind or prioritize wrong groups, | |
605 | use:: | |
606 | ||
607 | ceph pg cancel-force-recovery {pg-id} [{pg-id #2}] [{pg-id #3} ...] | |
608 | ceph pg cancel-force-backfill {pg-id} [{pg-id #2}] [{pg-id #3} ...] | |
609 | ||
610 | This will remove "force" flag from those PGs and they will be processed | |
611 | in default order. Again, this doesn't affect currently processed placement | |
612 | group, only those that are still queued. | |
613 | ||
614 | The "force" flag is cleared automatically after recovery or backfill of group | |
615 | is done. | |
7c673cae | 616 | |
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617 | Similarly, you may use the following commands to force Ceph to perform recovery |
618 | or backfill on all placement groups from a specified pool first:: | |
619 | ||
620 | ceph osd pool force-recovery {pool-name} | |
621 | ceph osd pool force-backfill {pool-name} | |
622 | ||
623 | or:: | |
624 | ||
625 | ceph osd pool cancel-force-recovery {pool-name} | |
626 | ceph osd pool cancel-force-backfill {pool-name} | |
627 | ||
628 | to restore to the default recovery or backfill priority if you change your mind. | |
629 | ||
630 | Note that these commands could possibly break the ordering of Ceph's internal | |
631 | priority computations, so use them with caution! | |
632 | Especially, if you have multiple pools that are currently sharing the same | |
633 | underlying OSDs, and some particular pools hold data more important than others, | |
634 | we recommend you use the following command to re-arrange all pools's | |
635 | recovery/backfill priority in a better order:: | |
636 | ||
637 | ceph osd pool set {pool-name} recovery_priority {value} | |
638 | ||
639 | For example, if you have 10 pools you could make the most important one priority 10, | |
640 | next 9, etc. Or you could leave most pools alone and have say 3 important pools | |
641 | all priority 1 or priorities 3, 2, 1 respectively. | |
642 | ||
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643 | Revert Lost |
644 | =========== | |
645 | ||
646 | If the cluster has lost one or more objects, and you have decided to | |
647 | abandon the search for the lost data, you must mark the unfound objects | |
648 | as ``lost``. | |
649 | ||
650 | If all possible locations have been queried and objects are still | |
651 | lost, you may have to give up on the lost objects. This is | |
652 | possible given unusual combinations of failures that allow the cluster | |
653 | to learn about writes that were performed before the writes themselves | |
654 | are recovered. | |
655 | ||
656 | Currently the only supported option is "revert", which will either roll back to | |
657 | a previous version of the object or (if it was a new object) forget about it | |
658 | entirely. To mark the "unfound" objects as "lost", execute the following:: | |
659 | ||
660 | ceph pg {pg-id} mark_unfound_lost revert|delete | |
661 | ||
662 | .. important:: Use this feature with caution, because it may confuse | |
663 | applications that expect the object(s) to exist. | |
664 | ||
665 | ||
666 | .. toctree:: | |
667 | :hidden: | |
668 | ||
669 | pg-states | |
670 | pg-concepts | |
671 | ||
672 | ||
673 | .. _Create a Pool: ../pools#createpool | |
674 | .. _Mapping PGs to OSDs: ../../../architecture#mapping-pgs-to-osds | |
675 | .. _pgcalc: http://ceph.com/pgcalc/ |